client.go/client_auth.go:
* add support for publickey key auth using the interface
  outlined by rsc in the previous auth CL

client_auth_test.go:
* password and publickey tests against server.go

common.go/server.go:
* move some helper methods from server.go into common.go
* generalise serializeRSASignature

R=rsc, agl, huin
CC=cw, golang-dev, n13m3y3r
https://golang.org/cl/5373055